Conventionally, curable compositions have been variously improved depending on the application, and have been widely used industrially as raw materials for optical components and formed articles, adhesives, coating agents, and the like.
In addition, the curable composition has also attracted attention as a composition for an optical element-fixing material such as an adhesive and a sealant for an optical element in producing an optical element sealant.
The optical elements are exemplified by a light emitter including various lasers such as a semiconductor laser (LD) and a light emitting diode (LED), a light-receiving element, a composite optical element, an optical integrated circuit, and the like.
In recent years, optical elements of blue light or white light having shorter emission peak wavelength have been developed and widely used. Luminance of such a light emitter having short emission peak wavelength has been dramatically enhanced, and accordingly, there has been tendency for a calorific value of the optical element to increase.
However, accompanying the increased luminance of the optical element in recent years, problems have occured that a cured product of the composition for the optical element-fixing material is exposed to heat at higher temperature caused by light or an optical element with higher energy for a long time, resulting in peeling due to deterioration and reduced adhesive strength.
In order to solve this problem, Patent Documents 1 to 3 propose compositions for optical element-fixing materials containing a polysilsesquioxane compound as a main component, and Patent Document 4 proposes a component for a semiconductor light-emitting device using a hydrolyzate/polycondensate of a silane compound, and the like.
However, even by the cured products such as the compositions and components described in Patent Documents 1 to 4, it was difficult to obtain peeling resistance or heat resistance while maintaining sufficient adhesive strength in some cases.
In addition, when a curable composition is applied in fixing an optical element or the like, a coating device having a discharge pipe (needle) as described in Patent Document 5 is normally used. In a coating device having such a discharge pipe, e.g. the discharge pipe vertically descends to approach the object to be coated, a predetermined amount of curable composition is discharged from the tip portion of the pipe, then the discharge pipe ascends to depart from the object to be coated, and the object laterally moves. Repeating this operation allows continuous and efficient coating with the curable composition.
However, in a case of using a highly viscous curable composition or the like, when the discharge pipe ascended, a part of the curable composition once discharged was raised in a form of string without breakage (cobwebbing) in some cases. Additionally, when the object to be coated laterally moved while the raised curable composition remained not broken, the curable composition sometimes adhered to a place other than a place to be rightly coated (resin splash), resulting in peripheral contamination.
This problem can be solved by lowering the viscosity of the curable composition. However, in this case, the discharged curable composition easily spreads, which often caused peripheral contamination.
Consequently, development of a curable composition which can provide a cured product excellent in adhesiveness, peeling resistance and heat resistance and be continuously applied without peripheral contamination (in the present invention, this property is referred to as “excellent workability in coating process), is strongly desired.